Roberto Accorsi

Coded aperture imaging

We have simulated the performance of various apertures used in coded aperture imaging—optically. The annulus, twin annulus, Fresnel zone plate and the uniformly redundant array have been decoded using a non-coherent correlation process. Ways of reducing the ‘d.c.’ background of the various apertures are discussed. Results of imaging extended and continuous-tone planar objects are presented.

Analytic determination of the resolution-equivalent effective diameter of a pinhole collimator

To account for photon penetration, the formulas used to calculate the geometric resolution of a pinhole collimator use an equivalent diameter d/sub e/ rather than the physical diameter of the aperture. The expression commonly used for d/sub e/, however, was originally derived to account for penetration in sensitivity calculations. In this paper, we show that the concept of equivalent diameter is also applicable to resolution calculations, propose angular-dependent expressions for d/sub e/ specific to resolution calculations, and discuss the limits of their applicability and how they compare to other expressions. Results show that for normal incidence Paixs expression for d/sub e/ tends to overestimate the resolution-equivalent diameter for full-width-at-half-maximum resolution, whereas Angers is a better approximation, but may produce underestimates for submillimeter resolution imagers, especially in the case of high-energy photons. For grazing incidence, both expressions may result in significant overestimates.

Optimization of a fully 3D single scatter simulation algorithm for 3D PET

We describe a new implementation of a single scatter simulation (SSS) algorithm for the prediction and correction of scatter in 3D PET. In this implementation, out of field of view (FoV) scatter and activity, side shields and oblique tilts are explicitly modelled. Comparison of SSS predictions with Monte Carlo simulations and experimental data from uniform, line and cold-bar phantoms showed that the code is accurate for uniform as well as asymmetric objects and can model different energy resolution crystals and low level discriminator (LLD) settings. Absolute quantitation studies show that for most applications, the code provides a better scatter estimate than the tail-fitting scatter correction method currently in use at our institution. Several parameters such as the density of scatter points, the number of scatter distribution sampling points and the axial extent of the FoV were optimized to minimize execution time, with particular emphasis on patient studies. Development and optimization were carried out in the case of GSO-based scanners, which enjoy relatively good energy resolution. SSS estimates for scanners with lower energy resolution may result in different agreement, especially because of a higher fraction of multiple scatter events. The algorithm was applied to a brain phantom as well as to clinical whole-body studies. It proved robust in the case of large patients, where the scatter fraction increases. The execution time, inclusive of interpolation, is typically under 5 min for a whole-body study (axial FoV: 81 cm) of a 100 kg patient.

A coded aperture for high-resolution nuclear medicine planar imaging with a conventional Anger camera: experimental results

Coded apertures have been investigated in the past for their promise of an improved signal-to-noise ratio (SNR) over pinhole and collimator systems. We describe a coded aperture camera designed for high-resolution (1.66-mm) imaging of low energy (140 keV) gamma-ray emitters with a conventional Anger camera. The aperture pattern was chosen to maximize the SNR and to allow a simple implementation of a new near-field artifact compensation technique based on the use of an antisymmetric array. Experimental results show that coded aperture imaging can produce good quality high-resolution planar images with high SNR while keeping exposure times and injected doses at reasonable levels.

Optimal number of pinholes in multi-pinhole SPECT for mouse brain imaging--a simulation study.

This study simulates a multi-pinhole single-photon emission computed tomography (SPECT) system using the Monte Carlo method, and investigates different multi-pinhole designs for quantitative mouse brain imaging. Prior approaches investigating multi-pinhole SPECT were not often optimal, as the number and geometrical arrangement of pinholes were usually chosen empirically. The present study seeks to optimize the number of pinholes for a given pinhole arrangement, and also for the specific application of quantitative neuroreceptor binding in the mouse brain. An analytical Monte Carlo simulation based method was used to generate the projection data for various count levels. A three-dimensional ordered-subsets expectation-maximization algorithm was developed and used to reconstruct the images, incorporating a realistic pinhole model for resolution recovery and noise reduction. Although artefacts arising from overlapping projections could be a major problem in multi-pinhole reconstruction, the cold-rod phantom study showed minimal loss of spatial resolution in multi-pinhole systems, compared to a single-pinhole system with the same pinhole diameter. A quantitative study of neuroreceptor binding sites using a mouse brain phantom and low activity (37 MBq) showed that the multi-pinhole system outperformed the single-pinhole system by maintaining the mean and lowering the variance in the measured uptake ratio. Multi-pinhole collimation can be used to reduce the injected dose and thereby reduce the radiation exposure to the animal. Results also suggest that the nine-pinhole configuration shown in this paper is a good choice for mouse brain imaging.

Optimal coded aperture patterns for improved SNR in nuclear medicine imaging

During a Nuclear Medicine project that called for the optimal design of a coded aperture we found that lowthroughput masks do not always provide a Signal-to-Noise Ratio (SNR) advantage. In this paper, we present the simulations of the performance of some coded aperture patterns chosen from different families and compare the results with theoretical predictions. A general expression for the SNR and its particular form for different patterns are provided. The choice of the optimal pattern family is discussed with reference to the characteristics of the object to be imaged and in light of the effect of near-field artifacts. No-Two-Holes-Touching (NTHT) arrays based on Modified Uniformly Redundant Arrays (MURAs) proved to offer the best compromise between SNR performance and practical fabrication constraints. r 2001 Elsevier Science B.V. All rights reserved.

Near-field artifact reduction in planar coded aperture imaging

Coded apertures for imaging problems are typically based on arrays having perfect cross-correlation properties. These arrays, however, guarantee a perfect point-spread function in far-field applications only. When these arrays are used in the near-field, artifacts arise. We present a mathematical derivation capable of predicting the shape of such artifacts. The theory shows that methods used in the past to compensate for the effects of background nonuniformities in far-field problems are also effective in reducing near-field artifacts. The case study of a nuclear medicine problem is presented to show good agreement of simulation and experimental results with mathematical predictions.

Improved Dose Regimen in Pediatric PET

PET image quality depends strongly on patient weight and habitus, decreasing for increasing weight and body mass index. Common adult injection rules prescribe either a dose proportional to weight or a fixed dose. In light patients, image quality may improve for decreasing weight more than by inverse proportion. If better quality than in average-adult studies does not justify the associated dose burden, attractive options are to reduce scan time, reduce dose, or any combination of the 2. The objective of this study was to determine quantitative injection rules for pediatric PET allowing clinical implementation of these trade-offs. Methods: Literature methods combining phantom with clinical data were followed to derive patient-specific noise-equivalent count rate density (NECRD) curves as a function of injected dose. From these, it was possible to estimate retrospectively for each patient the scan time that would have been sufficient for the same NECRD as in a 70-kg reference adult; the reduced dose sufficient for constant NECRD and scan time; and a general relationship among scan time, dose, and NECRD. Correlation to the patient statistic giving highest correlation, which was found to be weight, provided rules applicable prospectively. Data from 73 patients (weight, 11.5–91.4 kg; mean, 45.4 kg) were acquired and analyzed. Results: Following the clinical injection rule, which was proportional to weight, the NECRD increased linearly with decreasing weight. The expression exp[0.019 × (weight [kg] − 70)] for the time reduction possible with the current dose at constant NECRD correlated well with data (R2 = 0.86). The dose (in MBq) necessary for constant NECRD that should be injected 60 min before imaging is predicted well by 14.8 × exp[0.046 × weight (kg)] (R2 = 0.88) with the current scan time. A more complex expression to convert NECRD in whole or part to both dose and time savings was also derived. Comparison to common pediatric injection rules showed reasonable agreement with Clarks rule, albeit not at all weights. Conclusion: Results suggest that pediatric PET of constant image quality (in an NECRD sense) can be performed with time or dose savings, up to 50% for the lightest patients (10–20 kg).

Derivation and Validation of a Sensitivity Formula for Slit-Slat Collimation

An analytic formula is derived for the sensitivity of collimators achieving transverse collimation with a slit and axial collimation with a slat assembly whose septa may be parallel or focus on a line. The formula predicts sin3 phi dependence on the incidence angle and, in the particular case of parallel slats, 1/h dependence on the distance from the slit. More complex expressions for sensitivity that do not diverge at points near the slit or the focal line of the slat assembly are also derived. The predictions of the formulas are checked against simple cases for which solutions are available from direct calculation as well as against Monte Carlo simulation and published experimental data. Agreement is good in all cases analyzed. An approximate penetration model is also introduced: it involves the use of a sensitivity-effective slit width and septal length. Its predictions are compared to simulation results. Agreement was found to be compatible with statistical fluctuation (plusmn0.3%) for geometric sensitivity and better than 3 % of total sensitivity in the worst case of septa designed for high-energy (364.5 keV) photons.

Resolution- versus sensitivity-effective diameter in pinhole collimation: experimental verification

To account for photon penetration, the formulae used to calculate the geometric resolution of a pinhole collimator use an effective diameter d(e) rather than the physical diameter of the aperture. The expressions commonly used for d(e), however, were originally derived to include penetration in sensitivity calculations. To predict the full width at half maximum (FWHM) resolution of the point-spread function (PSF) of a knife-edge pinhole collimator, we have previously proposed simple expressions for a resolution-effective diameter d(re). Unlike those for d(e), expressions for d(re) predict both a dependence on the polar angle of the source (theta) and a non-isotropic PSF. In this paper, the new theory was tested by measuring experimentally the FWHM of the PSF. Results confirm the theoretical predictions that (a) d(re) provides the best estimates of the experimental FWHM as a function of theta and of the direction in the plane of the pinhole, (b) Paixs expression for d(e) tends to overestimate the FWHM, (c) Angers is a better approximation, but still cannot predict the dependence on theta, and (d) the FWHM decreases with decreasing theta, i.e. resolution improves for sources at the edge of the field-of-view.